Road condition estimation apparatus

ABSTRACT

The present invention is directed to a road condition estimation apparatus for estimating a road condition for use in a vehicle having a steering control unit for actuating a device mechanically independent of a manually operated steering member to steer at least a wheel of front and rear wheels. The apparatus includes an actuating signal detection unit for detecting an actuating signal for actuating the device of the steering control unit, an aligning torque estimation unit for estimating an aligning torque produced on the wheel on the basis of the actuating signal detected by the actuating signal detection unit, and a vehicle state variable detection unit for detecting a state variable of the vehicle. The apparatus further includes a wheel factor estimation unit for estimating at least one of wheel factors including a side force and a slip angle applied to the wheel on the basis of the vehicle state variable, and a grip factor estimation unit for estimating a grip factor of at least a tire of the wheel, in accordance with a relationship between the estimated aligning torque and the estimated wheel factor.

This application claims priority under 35 U.S.C. Sec. 119 toNo.2002-298354 filed in Japan on Oct. 11, 2002, the entire content ofwhich is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a road condition estimation apparatus,particularly relates to an apparatus for estimating a grip factorindicative of a grip level of tire on a road surface in a lateraldirection of a vehicle wheel, and/or estimating a coefficient offriction of a wheel to a road surface on the basis of the grip factor,to estimate a road condition on the basis of at least one of roadfactors including the grip factor and the coefficient of friction.

2. Description of the Related Arts

In order to maintain a stability of a vehicle, there is known heretoforean apparatus for controlling a braking force applied to each wheel onthe basis of vehicle state variable detected and determined, asdisclosed in Japanese Patent Laid-open Publication No.6-99800, forexample. In this publication, a target value of yaw rate is provided onthe basis of a vehicle speed and a steering angle, and an over steeringor an under steering is determined by a derived function of a differencebetween the actual value and the target value of the yaw rate. In caseof the over steering, a braking slip is increased on a front wheellocated outside of a curve when cornering, i.e., a braking force isincreased on the front wheel located outside of the curve. Whereas, incase of the under steering, the braking slip is increased on the frontwheel located inside of the curve. And, there is disclosed in JapanesePatent Laid-open Publication No.62-146754, an apparatus for setting afront wheel speed difference and a target value of lateral accelerationor yaw rate, on the basis of a steering angle and a vehicle speed, tocontrol a brake and/or an output of an engine.

In Japanese Patent Laid-open Publication No.11-99956, there is discloseda steering apparatus for a vehicle with a variable steering angle ratio,to prevent wheels from being steered excessively, wherein an index namedas a side force utilization ratio or lateral G utilization ratio isused. According to the apparatus as disclosed in the publication, a roadcoefficient of friction μ is estimated, to provide the side forceutilization ratio. It is described that reaction force of a rack axiswith the same steering angle applied by a road surface will be reducedin accordance with the road coefficient of friction μ, because the lowerthe road coefficient of friction μ is, the more a cornering power Cp oftire will be reduced. Therefore, it is concluded that the roadcoefficient of friction μ can be estimated by measuring the steeringangle of front wheels and the reaction force of the rack axis, andcomparing the reaction force of the rack axis against the steering angleof front wheels and a reference reaction force of the rack axis which isprovided in advance as an inside model. Moreover, an equivalent frictioncircle is provided on the basis of the road coefficient of friction μ,then an amount of friction force used by a longitudinal force issubtracted from it to provide a maximal side force to be produced, and aratio of the presently produced side force and the maximal side force isset as the side force utilization ratio. Or, a lateral G sensor may beprovided for setting the lateral G utilization ratio on the basis of thelateral G detected by the sensor.

Recent progress in electronics engineering has brought a so-called“by-wire system” into various manipulation systems for vehicles, such asa steer-by-wire system for use in a steering control system. Forexample, in Japanese Patent Laid-open Publication No.2001-191937, thereis disclosed the steer-by-wire system, wherein a steering angle iscontrolled in response to movement of a steering actuator operated by amanually operated steering member, e.g., steering wheel, which is notmechanically connected with wheels, and a steering control apparatus forvehicles has been proposed as an improvement of the system. Also, inJapanese Patent Laid-open Publication No.7-329808, there is disclosed asteering control apparatus for controlling a steering angle for rearwheels by means of a motor, which may be included in the field of thesteer-by-wire system.

In the case where friction between a road surface and a vehicle tire hascome to its limit, to cause an excessive under steering condition, it isrequired not only to control a yawing motion of the vehicle, i.e., aposition of the vehicle on the road surface, but also to reduce thevehicle speed, in order to maintain a radius of cornering curve of thevehicle as intended by the vehicle driver. According to the apparatus asdisclosed in the Publication No.6-99800, however, the vehicle behavioris determined after the tire reached the friction limit. When thevehicle speed is reduced in that situation, therefore, the corneringforce will be reduced, whereby the tendency of under steering might beincreased. Furthermore, according to the actual control system, as thereis provided a dead zone for a control, the control generally beginsafter a certain vehicle behavior occurred.

As the curve of a vehicle road is formed into a clothoid curve, when thevehicle driver intends to trace the curve of the road, the steeringwheel will be rotated with a gradually increasing amount. In the casewhere the vehicle speed is high when the vehicle has entered into thecurve, therefore, the side force produced on the wheel will not balancewith a centrifugal force, whereby the vehicle tends to be forced outsideof the curve. In those cases, the apparatuses as disclosed in thePublication No.6-99800 and 62-146754 will operate to control the motionof the vehicle. However, as the controls begin at the cornering limit,the vehicle speed may not be reduced sufficiently by those controls.Therefore, it might be caused that the vehicle can not be prevented onlyby those controls from being forced outside of the curve.

Furthermore, it is disclosed in AUTOMOTIVE ENGINEERING HANDBOOK, FirstVolume, for BASIC & THEORY, issued on Feb. 1, 1990 by Society ofAutomotive Engineers of Japan, Inc., Pages 179 and 180, such a statethat a tire rotates on a road, skidding at a slip angle a, as shown in apart of FIG. 1 of the present application. As indicated by broken linesin FIG. 1, a tread surface of the tire contacts a road surface at afront end of the contacting surface including Point (A) in FIG. 1, andmoves with the tire advanced, being adhesive to the road surface up toPoint (B). The tire begins to slip when a deformation force by a lateralshearing deformation has become equal to a friction force, and departsfrom the road surface at a rear end including Point (C). In this case, aside force Fy produced on the overall contacting surface equals to aproduct of a deformed area of the tread in its lateral direction (asindicated by a hutching area in FIG. 1) multiplied by its lateralelastic coefficient per unit area. As shown in FIG. 2, a point ofapplication of force for the side force Fy is shifted rearward (leftwardin FIG. 1) from a point (O) on the center line of the tire, by adistance (e_(n)) which is called as a pneumatic trail. Accordingly, amoment Fy·e_(n) becomes an aligning torque (Tsa), which acts in such adirection to reduce the slip angle α, and which may be called as aself-aligning torque.

Next will be explained the case where the tire is installed on avehicle, with reference to FIG. 2 which simplified FIG. 1. With respectto steered wheels of a vehicle, in general, a caster angle is providedso that a steering wheel can be returned to its original positionsmoothly, to produce a caster trail (e_(c)). Therefore, the tirecontacts the road surface at a point (O′), so that the moment forforcing the steering wheel to be positioned on its original positionbecomes Fy·(e_(n)+e_(c)). When a lateral grip force of the tire isreduced to enlarge the slip area, the lateral deformation of the treadwill result in changing a shape of ABC in FIG. 2 into a shape of ADC. Asa result, the point of application of force for the side force Fy willbe shifted forward in the advancing direction of the vehicle, from Point(H) to Point (J). That is, the pneumatic trail (e_(n)) will be reduced.Therefore, even in the case where the same side force Fy acts on thetire, if the adhesive area is relatively large and the slip area isrelatively small, i.e., the lateral grip force of the tire is relativelylarge, the pneumatic trail (e_(n)) will be relatively large, so that thealigning torque Tsa will be relatively large. On the contrary, if thelateral grip force of the tire is lessened, and the slip area isenlarged, then the pneumatic trail (e_(n)) will become relatively small,so that the aligning torque Tsa will be reduced.

As described above, by monitoring the variation of the pneumatic trail(e_(n)), the grip level of the tire in its lateral direction can bedetected. And, the variation of the pneumatic trail (e_(n)) results inthe aligning torque Tsa, on the basis of which can be estimated a gripfactor indicative of a grip level of the tire in its lateral direction,with respect to a front wheel for example (hereinafter simply referredto as grip factor). With respect to the grip factor, it can be estimatedon the basis of a margin of side force for road friction, as describedlater in detail.

The grip factor as described above is clearly distinguished from theside force utilization ratio, or lateral G utilization ratio asdescribed in the Publication No.11-99956, wherein the maximal side forcewhich can be produced on the road surface is obtained on the basis ofthe road coefficient of friction μ. And, this road coefficient offriction μ is estimated on the basis of a reliability of the corneringpower Cp (value of the side force per the slip angle of one degree) onthe road coefficient of friction A. However, the cornering power Cprelies not only on the road coefficient of friction μ, but also a shapeof the area of the road contacting the tire (its contacting length andwidth to the road), and elasticity of the tread rubber. For example, inthe case where water exists on the tread surface, or the case where theelasticity of the tread rubber has been changed due to wear of the tireor its temperature change, the cornering power Cp will vary, even if theroad coefficient of friction μ is constant. In the PublicationNo.11-99956, therefore, nothing has been considered about thecharacteristic of the tire which constitutes the wheel.

On the contrary, if the grip factor as described before is used directlyfor the various controls, they can be achieved appropriately inaccordance with a road condition, at the early stage well before thefriction between the road surface and the tire comes to its limit. Inaddition, the coefficient of friction of the road surface can beestimated on the basis of the grip factor, as will be described later indetail. Therefore, if the grip factor and the coefficient of frictionare employed as the road factors, to estimate the road condition on thebasis of the road factors, the road condition can be estimated at theearly stage well before the friction between the road surface and thetire comes to its limit. Especially when the steer-by-wire system asdisclosed in the Publication Nos.2001-191937 and 7-329808 is employed,the steering control is made by actuating means (e.g., motor) which isseparated mechanically from the steering wheel served as the manuallyoperated steering member. In this case, therefore, the aligning torqueas described before can be obtained by detecting a signal (e.g.,electric current) for driving the actuating means, as will be describedlater in detail, so that the estimation of the grip factor can be madeeasily.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a roadcondition estimation apparatus for use in a vehicle with a steer-by-wiresystem which is adapted to control a steering angle by an actuatormechanically separated from a manually operated steering member, andcapable of accurately estimating a condition of a road surface at anappropriate timing, when the vehicle is running on the road surface.

It is another object of the present invention to provide a vehiclemotion control apparatus for use in a vehicle with a steer-by-wiresystem which is adapted to control a steering angle by an actuatorseparated mechanically from a manually operated steering member, andcapable of accurately estimating a condition of a road surface at anappropriate timing, when the vehicle is running on the road surface, andachieving a motion control appropriately on the basis of the estimatedroad condition.

In accomplishing the above and other objects, the road conditionestimation apparatus is provided for estimating a road condition for usein a vehicle having steering control means for actuating a devicemechanically independent of a manually operated steering member to steerat least a wheel of front and rear wheels. The apparatus includesactuating signal detection means for detecting an actuating signal foractuating the device of the steering control means, aligning torqueestimation means for estimating an aligning torque produced on the wheelon the basis of the actuating signal detected by the actuating signaldetection means, vehicle state variable detection means for detecting astate variable of the vehicle, wheel factor estimation means forestimating at least one of wheel factors including a side force and aslip angle applied to the wheel on the basis of the state variabledetected by the vehicle state variable detection means, and grip factorestimation means for estimating a grip factor of at least a tire of thewheel, in accordance with a relationship between the aligning torqueestimated by the aligning torque estimation means and the wheel factorestimated by the wheel factor estimation means.

In the steering control means as described above, the device may beconstituted by a motor, and therefore the actuating signal for actuatingthe device may be electric current. As for the state variable, factorsrelating to the vehicle in motion are employed, such as vehicle speed,lateral acceleration, yaw rate, steering angle, and the like.

Preferably, the apparatus further includes friction estimation means forestimating a coefficient of friction of a road on which the vehicle isrunning, on the basis of the grip factor estimated by the grip factorestimation means.

Furthermore, the apparatus may include warning means for warning to avehicle driver when at least one of road factors including the gripfactor estimated by the grip factor estimation means and the coefficientof friction estimated by the friction estimation means is less than apredetermined value.

As for a vehicle motion control apparatus, it is preferably providedwith an apparatus for estimating a road condition for use in a vehiclehaving steering control means for actuating a device mechanicallyindependent of a manually operated steering member to steer at least awheel of front and rear wheels. And, the vehicle motion controlapparatus includes actuating signal detection means for detecting anactuating signal for actuating the device of the steering control means,aligning torque estimation means for estimating an aligning torqueproduced on the wheel on the basis of the actuating signal detected bythe actuating signal detection means, vehicle state variable detectionmeans for detecting a state variable of the vehicle, wheel factorestimation means for estimating at least one of wheel factors includinga side force and a slip angle applied to the wheel on the basis of thestate variable detected by the vehicle state variable detection means,and grip factor estimation means for estimating a grip factor of atleast a tire of the wheel, in accordance with a relationship between thealignment torque estimated by the aligning torque estimation means andthe wheel factor estimated by the wheel factor estimation means. Thevehicle motion control apparatus further includes control means forperforming at least one of a steering control to front wheels of thevehicle, a steering control to rear wheels of the vehicle and a brakingforce control to each wheel of the vehicle, on the basis of the gripfactor estimated by the grip factor estimation means.

The vehicle motion control may further include friction estimation meansfor estimating a coefficient of friction of a road on which the vehicleis running, on the basis of the grip factor estimated by the grip factorestimation means, and the control means may perform at least one of thesteering control to front wheels of the vehicle, the steering control torear wheels of the vehicle and the braking force control to each wheelof the vehicle, on the basis of at least one of road factors includingthe grip factor estimated by the grip factor estimation means and thecoefficient of friction estimated by the friction estimation means.

In the above vehicle motion control apparatus, the control means mayprovide parameters for at least one of the steering control to frontwheels of the vehicle, the steering control to rear wheels of thevehicle and the braking force control to each wheel of the vehicle, onthe basis of at least one of the road factors. Or, the control means mayprovide a characteristic of steering amount of wheel in accordance withthe amount of steering operation of a vehicle driver, and may provide acharacteristic of a desired braking force in accordance with the amountof braking operation of the vehicle driver, on the basis of at least oneof the road factors.

BRIEF DESCRIPTION OF THE DRAWINGS

The above stated object and following description will become readilyapparent with reference to the accompanying drawings, wherein likereferenced numerals denote like elements, and in which:

FIG. 1 is a diagram showing a relationship between aligning torque andside force, when a tire is advanced, skidding in a lateral direction;

FIG. 2 is a diagram simplifying the relationship between aligning torqueand side force as shown in FIG. 1;

FIG. 3 is a schematic block diagram of a vehicle motion controlapparatus according to an embodiment of the present invention;

FIG. 4 is a block diagram illustrating systems of a motion controlapparatus according to an embodiment of the present invention;

FIG. 5 is a block diagram of a steering control unit in a steeringcontrol system of a vehicle motion control apparatus according to anembodiment of the present invention;

FIG. 6 is a block diagram of a steering control system of a vehiclemotion control apparatus according to an embodiment of the presentinvention;

FIG. 7 is a block diagram of a grip factor estimation apparatusaccording to an embodiment of the present invention;

FIG. 8 is a block diagram of a grip factor estimation unit according toan embodiment of the present invention;

FIG. 9 is a diagram showing a relationship between aligning torque andside force according to an embodiment of the present invention;

FIG. 10 is a diagram showing a relationship between aligning torque andside force to wheel slip angle according to an embodiment of the presentinvention;

FIG. 11 is a diagram showing a relationship between aligning torque andwheel slip angle according to an embodiment of the present invention;

FIG. 12 is a diagram showing a relationship between aligning torque andwheel slip angle according to an embodiment of the present invention;

FIG. 13 is a diagram showing a relationship between aligning torque andwheel slip angle according to an embodiment of the present invention;

FIG. 14 is a diagram showing a relationship between aligning torque andwheel slip angle according to an embodiment of the present invention;

FIG. 15 is a diagram showing a characteristic of frictional torqueresulted from the Coulomb's friction for use in correcting the aligningtorque to be estimated, according to an embodiment of the presentinvention;

FIG. 16 is a diagram showing a characteristic of friction component ofsteering system for use in correcting the aligning torque to beestimated, according to an embodiment of the present invention;

FIG. 17 is a block diagram for estimating a wheel factor on the basis ofa steering angle of a wheel and a vehicle speed, by means of an observerbased on a vehicle model, according to an embodiment of the presentinvention;

FIG. 18 is a block diagram for estimating a wheel factor on the basis ofan observer based on a vehicle model, with a correction added thereto,according to an embodiment of the present invention;

FIG. 19 is a block diagram for calculating a wheel factor directly,through a state variable calculation, without using an observer,according to an embodiment of the present invention;

FIG. 20 is a diagram showing a relationship between aligning torque andwheel slip angle according to an embodiment of the present invention;

FIG. 21 is a diagram showing a relationship between a grip factor εbased on a pneumatic trail and a grip factor ε m based on a margin ofside force for road friction, according to the present invention;

FIG. 22 is a block diagram showing a road coefficient of frictionestimation apparatus, as an example of a road condition estimationapparatus according to an embodiment of the present invention;

FIG. 23 is a block diagram showing an example for estimating a roadcoefficient of friction on the basis of an aligning torque and a wheelfactor in a road condition estimation apparatus according to anembodiment of the present invention;

FIG. 24 is a diagram showing an example for estimating a roadcoefficient of friction, with side force used as wheel factor, in a roadcoefficient of friction estimation apparatus according to an embodimentof the present invention;

FIG. 25 is a diagram showing an example for estimating a roadcoefficient of friction, with a wheel slip angle used as a wheel factor,in a road coefficient of friction estimation apparatus according to anembodiment of the present invention;

FIG. 26 is a diagram showing a relationship between wheel slip angle andaligning torque, in a road coefficient of friction estimation apparatusaccording to an embodiment of the present invention;

FIG. 27 is a block diagram for setting a desired front wheel angle in asteer-by-wire system according to an embodiment of the presentinvention;

FIG. 28 is a block diagram for setting a desired rear wheel angle in asteer-by-wire system according to an embodiment of the presentinvention;

FIG. 29 is a block diagram for setting a desired steering reaction forcein a steering reaction force simulator according to an embodiment of thepresent invention;

FIG. 30 is a block diagram for setting a desired braking force for eachwheel in a vehicle motion control apparatus according to an embodimentof the present invention; and

FIG. 31 is a diagram showing a desired braking force for each wheel in avehicle motion control apparatus according to an embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 3-6, there is schematically illustrated a vehiclewith a motion control apparatus having a road condition estimationapparatus according to an embodiment of the present invention. Thevehicle of the present embodiment is structured as shown in FIG. 3, andconstituted as shown in FIG. 4 by a front wheel steering control system(FSTR), rear wheel steering control system (RSTR), steering reactionforce simulator (SST), brake control system (BRK), throttle controlsystem (SLT), shift control system (ATM), and warning system (ALM),which are connected with each other through a communication bus, so thateach system may hold each information commonly.

As shown in FIG. 3, at front wheels WH1 and WH2, and rear wheels WH3 andWH4 of the vehicle, there are provided wheel speed sensors WS1 to WS4respectively, which are connected to the electronic controller ECU, andby which a signal having pulses proportional to a rotational speed ofeach wheel, i.e., a wheel speed signal is fed to the electroniccontroller ECU. Furthermore, there are provided a brake switch BS whichturns on when the brake pedal BP is depressed, and turns off when thebrake pedal BP is released, steering angle sensor FS for detecting asteering angle θf of the front wheels WH1 and WH2, a steering anglesensor RS for detecting a steering angle θr of the rear wheels WH3 andWH4, a longitudinal acceleration sensor XG for detecting a vehiclelongitudinal acceleration Gx, a lateral acceleration sensor YG fordetecting a vehicle lateral acceleration Gy, a yaw rate sensor YR fordetecting a yaw rate γ of the vehicle, and so on, which are electricallyconnected to the electronic controller ECU.

According to the present embodiment, the steering control systemincludes the front wheel steering control system (FSTR) and rear wheelsteering control system (RSTR), as described above, so that it is soconstituted that any one of the front wheels WH1 and WH2, and rearwheels WH3 and WH4 are controlled to be steered by an actuator, which isseparated mechanically from a steering wheel SW served as a manuallyoperated steering member. That is, as shown in FIG. 3, the steeringwheel SW and the front wheels WH1 and WH2 are not connectedmechanically. Therefore, the operation of the steering wheel SW isdetected at a steering operation detection unit SS, which may beconstituted by a steering (wheel operation) angle sensor SA (in FIG. 4),steering (wheel operation) torque sensor TS, or the like. The rear wheelsteering control system (RSTR) as shown in FIG. 3 may be omitted. Or,the front wheel steering control system may be constituted by aconventional mechanically connected system, and only the rear wheelsteering control system may be adapted to perform the steering controlby the actuator which is separated mechanically from a manually operatedsteering member.

In the front wheel steering control system (FSTR), a steering controlunit ECU1 which is provided with CPU, ROM and RAM for the front steeringcontrol, and to which a turning angle sensor RS and an electric currentsensor ES are connected, and a motor MF is connected through a drivecircuit DC1. In operation, a desired steering angle (target value) θftfor the steering angle of each wheel is provided in the steering controlunit ECU1, on the basis of the amount of steering operation by thedriver detected at the steering operation detection unit SS as shown inFIG. 3, and vehicle state variable (vehicle speed, yaw rate,longitudinal acceleration, lateral acceleration, or the like),frictional state between each wheel and the road surface and so on. Onthe basis of the desired steering angle θft, therefore, the motor MF forthe front wheel steering control is actuated to control the steeringangle θf for the front wheels.

Furthermore, the steering reaction force simulator (SST) is controlledto provide appropriate reaction force, in accordance with the vehiclestate when running, or the state of the steering wheel SW when beingoperated. As shown in FIG. 3, the steering reaction force simulator(SST) is operatively connected to the steering wheel SW, and, as shownin FIG. 4, the turning angle sensor RS and electric current sensor ESare connected to the control unit ECU3, to which a reaction motor RM isconnected through a drive circuit DC3. Accordingly, a torque can beproduced by the reaction motor RM and a resilient member (not shown) forproducing force applied to the steering wheel SW in a direction forpositioning it to hold the vehicle moving straight forward, as disclosedin the publication No.2001-191937.

The steering control unit for use in the front wheel steering controlsystem (FSTR) is constituted as shown in FIG. 5. As components of thesteering control unit for use in the rear wheel steering control system(RSTR) are substantially the same as those for use in the front wheelsteering control system (FSTR), counterparts of the latter componentsare indicated in the parentheses followed by the latter components, inthe following explanation. The information about the state of steeringoperation by the driver and the moving vehicle state (vehicle speed, yawrate, longitudinal acceleration, lateral acceleration, vehicle slipangle, or the like) is fed through a communication bus into the steeringcontrol unit ECU1 (ECU2), where the desired value of the steering angle(θft for the front wheel) is calculated. And, on the basis of thedesired steering angle, the motor MF (MR) is actuated through a drivecircuit 22. As for the motor MF (MR), a brushless DC motor may beemployed, and a rotation angle sensor KSF (KSR) is operatively mountedon it. However, the motor MF (MR) is not limited to that DC motor, butmay be employed those of other types. In operation, the motor MF (MR) isactuated to be controlled in response to the signal detected by therotation angle sensor KSF (KSR). Then, reaction torque to the roadsurface can be estimated on the basis of electric current detected by acurrent detection section 23 which is provided in the drive circuit 22.In FIG. 5, 24 indicates an interface, 25 indicates a constant voltageregulator, and 10 indicates a power source.

With respect to the motor MF in the front wheel steering control system(FSTR), it is controlled as shown in FIG. 6, for example. At the outset,on the basis of the state of steering operation by the driver and themoving vehicle state, the desired steering angle θft (θrt) for thewheels WH1 and WH2 (wheels WH3 and WH4) is calculated at a unit 41.Then, a limit to the desired steering angle is provided in accordancewith the vehicle speed for the sake of fail safe, at a unit 42. And, thePD control is performed on the basis of a deviation between the desiredsteering angle θft (θrt) and the actual steering angle θfa (θra) atunits 43-54, so that the motor MF (MR) is actuated in accordance withthe duty ratio for the PWM control, which is calculated on the basis ofthe steering angle deviation. As each unit is constituted insubstantially the same manner as disclosed in the publicationNo.7-329808, its explanation is omitted.

Next, the brake control system (BRK) according to the present embodimentis constituted by a so-called brake-by-wire system. As shown in FIG. 3,wheel speed sensors WS1-WS4 are operatively mounted on the wheelsWH1-WH4, and pressure sensors PS1-PS4 are provided for detecting thepressure of the wheel brake cylinders (not shown). Then, as shown inFIG. 4, the pressure sensors PS1-PS4 are connected to a brake controlunit ECU4, to each of which a solenoid valve SL is connected through asolenoid drive circuit DC4. And, the requirement for braking the vehicleby the driver is detected at the braking amount detection unit includinga brake pedal stroke sensor (not shown) or the like. The pressure in awheel brake cylinder of each wheel is controlled on the basis of theamount of operation of the brake pedal BP by the vehicle driver, themoving vehicle state, the frictional state between the wheel and theroad surface, and so on. The brake control system of the presentembodiment is adapted to perform the anti-skid control (ABS), brakeassist control (BA), traction control (TRC), vehicle stability control(VSC), and adaptive cruise control (ACC).

According to the present embodiment, an engine EG is an internalcombustion engine which is provided with a fuel injection apparatus FIand a throttle control apparatus TH, which is served as the throttlecontrol system (SLT), and which is adapted to provide a desired throttleopening in response to operation of an accelerator pedal AP, andactuated in response to an output signal of an electronic controller ECUto control the throttle control apparatus TH, and actuate the fuelinjection apparatus FI to control the injected fuel. As shown in FIG. 4,a position sensor POS is connected to a throttle control unit ECU5, towhich a throttle control actuator AC5 is connected through a drivecircuit DC5. According to the present embodiment, the engine EG isoperatively connected with the rear wheels WH3 and WH4 through atransmission GS and differential gear DF to provide a so-calledrear-drive system, but the present embodiment is not limited to therear-drive system.

The shift control system (ATM) includes a shift control unit ECU6 forthe shift control of the automatic transmission, to which a shiftcontrol actuator AC6 is connected through a drive circuit DC6. Thewarning system (ALM) is adapted to output a warning signal when theestimated grip factor is less than a predetermined value, and includes awarning control unit ECU7, to which a warning apparatus AC7 forproviding the warning information through an indicator or audio systemor the like. Those control units ECU1-ECU7 are connected to thecommunication bus through a communication unit provided with CPU, ROMand RAM for the communication, respectively. Accordingly, theinformation required for each control system can be transmitted by othercontrol systems.

FIG. 7 shows a grip factor estimation apparatus according to anembodiment of the present invention, which is based upon the fact thatthe reaction torque of the wheel applied by the road surface can beestimated by detecting a signal (electric current) for actuating themotor MF (MR), because the electric current for actuating the motor MF(MR) is proportional to an output torque. As the estimated reactiontorque includes the components resulted from the friction of the membersin the steering system, the reaction torque estimated by the electriccurrent for actuating the motor MF (MR) is compensated by the componentsresulted from the friction of the members in the steering system, toobtain the aligning torque Tsa. On the basis of the relationship betweenthe aligning torque Tsa and the wheel factors indicated by the wheelslip angle or side force, the grip factor E indicative of the grip stateof the wheel against the road surface can be estimated.

Referring to FIG. 7, the electric current for actuating the motor MF(MR) under the steering control is detected at a current detection unitM1 (e.g., current detection section 23 in FIG. 5), and the reactiontorque is estimated at a reaction torque estimation unit M2 on the basisof the detected result of the current detection unit M1. Also, thesteering angle of the wheel is detected at a steering angle of wheeldetection unit M3, then, on the basis of the detected steering angle ofthe wheel, the steering friction torque corresponding to the frictioncomponent of the members in the steering system is estimated at asteering friction torque estimation unit M4. On the basis of theestimated reaction torque and steering friction torque, aligning torqueis estimated at an aligning torque estimation unit M5. On the otherhand, on the basis of the vehicle speed detected at a vehicle speeddetection unit M6, the vehicle behavior detected at a vehicle behaviordetection unit M7, and the steering angle of the wheel detected at thesteering angle of wheel detection unit M3, at least one of the wheelfactor out of the wheel factors Wx including the side force Fy to thewheel and the wheel slip angle α is estimated at a wheel factorestimation unit M8. Then, on the basis of the variation of the aligningtorque estimated at the aligning torque estimation unit M5 to the wheelfactor estimated at the wheel factor estimation unit M8, the grip factorε of the wheel is estimated at a grip factor estimation unit M10.

FIG. 8 shows an example of the grip factor estimation unit M10, whereinthe grip factor ε of the wheel is estimated on the basis of the aligningtorque and wheel factor (side force Fy, or wheel slip angle α). On thebasis of the aligning torque Tsa estimated at the aligning torqueestimation unit M5 and the wheel factor Wx of the side force Fy or thewheel slip angle α estimated at the wheel factor estimation unit M8, agradient (K) of the aligning torque in the vicinity of the origin(abbreviated as origin gradient), i.e., the origin gradient (K) of thealigning torque to the wheel factor Wx is estimated at an origingradient estimation unit M11. On the basis of the origin gradient (K), areference aligning torque which provides for a wheel in a state almostcompletely gripped in its lateral direction is set at a unit M12. Then,the grip factor ε is obtained at a grip factor calculation unit M13, onthe basis of the actual aligning torque obtained at the aligning torqueestimation unit M5 and the reference aligning torque as set above.

Referring to FIG. 9, an example for estimating the grip factor ε whenthe side force Fy is used as the wheel factor Wx will be explainedhereinafter. At the outset, the aligning torque Tsa to the side force Fyis of the characteristic as indicated by “Tsaa” in FIG. 9. As describedbefore, if the actual aligning torque is indicated by “Tsaa” and theside force is indicated by “Fy”, “Tsaa” can be obtained by the equationof Tsaa=Fy·(e_(n)+e_(c)), so that the nonlinear characteristic of theactual aligning torque Tsaa to the side force Fy corresponds to thelinear variation of the pneumatic trail e_(n). Therefore, a gradient(K1) of the actual aligning torque Tsaa in the vicinity of the origin(0), where the front wheel is in the gripped state, to the side force Fyis identified, to obtain the characteristic of the aligning torque inthe completely gripped state, i.e., reference aligning torque Tsao. Withrespect to the gradient (K1) corresponding to the origin gradient of thealigning torque, a predetermined value is set as an initial value at theoutset, then it is corrected by identifying the gradient (K1) while thevehicle is running at an approximately constant speed, with the periodof acceleration or deceleration of the vehicle eliminated.

As the pneumatic trail en varies in response to the gripped state of thewheel, the reference aligning torque Tsao indicative of the state of thewheel which is almost completely gripped in its lateral direction can beset by the gradient (K1) in the vicinity of the origin, where the wheelis in such a state as almost completely gripped in its lateral direction(state of moving straight forward), to provide Tsao=K1·Fy. Then, thegrip factor ε can be obtained by the ratio of the reference aligningtorque Tsao and the actual aligning torque Tsaa. For example, when theside force is “Fy1”, the reference aligning torque Tsao is set as“Tsao1” (=K1·Fy1), and the actual aligning torque Tsaa is obtained as“Tsaa1”, so that the grip factor ε is obtained by ε=Tsaa1/Tsao1.

Next, an example for estimating the grip factor ε when the wheel slipangle α is used as the wheel factor Wx will be explained hereinafter. Atthe outset, the aligning torque Tsa to the wheel slip angle α is of thecharacteristic as indicated in FIG. 10. In the same manner as the sideforce was used as the wheel factor, the reference aligning torque in thealmost completely gripped state will be of the nonlinear characteristicto the wheel slip angle, as indicated by “Tsar” in FIG. 11. Thisnonlinear characteristic depends on the road coefficient of friction μ.In order to set the reference aligning torque, therefore, it will berequired to estimate the road coefficient of friction μ. However, it isdifficult to estimate the road coefficient of friction μ, because thealigning torque Tsa will not be varied so much by the road coefficientof friction μ, in the state where the grip factor is relatively high,i.e., with the wheel gripped at a relatively small slip angle, asdescribed before. In this case, therefore, the grip factor is estimatedby approximating the reference aligning torque to the linearcharacteristic, as shown in FIG. 12. That is, a gradient (K2) of thealigning torque Tsa in the vicinity of the origin of the wheel slipangle α, to the wheel slip angle α is obtained, to provide a referencealigning torque Tsas as Tsas=K2·α. Then, the grip factor ε can beobtained by the ratio of the reference aligning torque Tsas and theactual aligning torque Tsaa. For example, when the side force is “α1”,the reference aligning torque is set as “Tsas1” (=K2·α1), and the gripfactor ε is obtained by ε=Tsaa1/Tsas1.

According to the method by approximating the reference aligning torqueto the linear characteristic, as shown in FIG. 12, the accuracy inestimating the grip factor might be lessened in such an area that thewheel slip angle a is relatively large. In this case, therefore, thegradient of aligning torque may be set to “K3” as shown in FIG. 13, whenthe wheel slip angle exceeds a predetermined slip angle, and thenonlinearity of the reference aligning torque may be approximated to astraight line of “OMN” in FIG. 13. In this case, it is preferable thatthe gradient of aligning torque K3 is obtained in advance by anexperiment, and identified to correct it while the vehicle is running.In FIG. 13, the point (M) may be set on the basis of the inflectionpoint (P) of the actual aligning torque. This is because the roadcoefficient of friction μ can be estimated on the basis of an inflectionpoint of the aligning torque. Therefore, after the inflection point (P)of the actual aligning torque Tsaa is obtained, the wheel slip anglewhich is larger than the slip angle corresponding to the inflectionpoint (P) by a predetermined value is set as the point (M), to changethe gradient of the aligning torque from K2 to K3.

Furthermore, as the reference aligning torque to the slip angle isaffected by the road coefficient of friction μ, the reference aligningtorque characteristic may be set at high accuracy by setting thereference aligning torque on the basis of the inflection point (P) ofthe actual aligning torque Tsaa as shown in FIG. 14. For example, whenthe road coefficient of friction μ is reduced, the characteristic of theactual aligning torque Tsaa is changed from a rigid line to a brokenline as shown in FIG. 14. In other words, if the road coefficient offriction μ is reduced, the inflection point of the actual aligningtorque Tsaa is changed from the point (P) to a point (P′). Therefore,the reference aligning torque characteristic (Tsat) is required tochange “OMN” to “OM′N′”. In this case, the point (M′) is set on thebasis of the inflection point (P′), even if the road coefficient offriction L is changed, the reference aligning torque characteristic canbe set in accordance with the change of the road coefficient of frictionμ.

Consequently, as shown in FIG. 14, the reference aligning torquecharacteristic can be accurately approximated to the one for thecomplete gripped state by setting the reference aligning torque as Tsatand Tsat′ on the basis of the inflection points (P) and (P′) of theactual aligning torque Tsaa and actual aligning torque Tsaa′.Furthermore, it is possible to set a point for altering the aligningtorque gradient, in accordance with the road coefficient of frictionwhich is estimated according to a method for estimating it on the basisof the grip factor as described later.

As described before, in order to obtain the estimated reaction torqueaccurately, it is required to correct the reaction torque estimated bythe electric current for actuating the motor MF (MR), on the basis ofthe frictional components in the steering system, as will explainedhereinafter with reference to FIGS. 15 and 16. FIG. 15 shows a methodfor obtaining the frictional components resulted from the Coulomb'sfriction in the steering system. At the outset, the steering angle ofthe wheel is increased as shown in the upper section of FIG. 15, thenthe reaction torque is detected just before the wheel is returned towardits original position (reaction torque Tx at the position “X” in thelower section in FIG. 15). Next, the steering angle of the wheel isdecreased as shown in the upper section of FIG. 15, and the reactiontorque Ty is detected at a position, where a varying amount of thereaction torque is changed against a variation of the steering angle(position “Y” in the lower section in FIG. 15). Then, the torque Ty issubtracted from the torque Tx to provide a frictional torque in thesteering system. This calculation is repeated every steering operation,and an average of the results obtained by a plurality number ofcalculations is used as the frictional torque.

Next, the correction by the frictional torque in the steering systemwill be explained with reference to FIG. 16. The relationship betweenthe reaction torque and the aligning torque has a hysteresis asindicated by the one-dotted chain line in FIG. 16. As for the frictionaltorque in the steering system, the value obtained as shown in FIG. 15 isused, and a gradient of the aligning torque Tsa against the reactiontorque Tstr is set as “1”. When the vehicle is running along a straightlane, the reaction torque Tstr is zero. When the driver starts thesteering operation to begin turning the steering wheel and the steeringangle of the wheel begins to be increased, the reaction torque Tstr willbe produced. First, the torque for compensating the Coulomb's frictionwill be produced, then the wheels (tires) will be turned to produce thealigning torque. Therefore, in the initial period for changing from thestate where the vehicle is running along the straight lane to the statewhere the steering operation is performed (within a range of hysteresiscaused by the frictional torque), the aligning torque has not beenproduced yet, with the reaction torque increased, as indicated by 0-A inFIG. 16. As a result, the estimated aligning torque will be output asthe actual aligning torque Tsaa (this is in fact the estimated valuewith the correction made, but the word of “estimated” is omittedherein), with a slight gradient to the reaction torque. With thesteering wheel turned (or rotated) further, if the reaction torqueexceeds the friction torque area, the actual aligning torque Tsaa willbe output along A-B in FIG. 16. If the steering wheel is returned towardits original position, so that the reaction torque is reduced, then theactual aligning torque Tsaa will be output along B-C in FIG. 16, with aslight gradient to the reaction torque. And, if the reaction torqueexceeds the friction torque area, the actual aligning torque Tsaa willbe output along C-D in FIG. 16, in the same manner as the steering wheelis turned further.

FIGS. 17-19 show an embodiment for estimating the wheel factor Wx (sideforce Fy or wheel slip angle α in the present embodiment). FIG. 17 showsan embodiment for estimating the wheel factor on the basis of thesteering angle of the wheel and the vehicle speed, by means of anobserver 61 based on a vehicle model, which is indicated by a vehicleparameter such as a vehicle state equation and wheel base, a parameterindicative of a tire property, and the like. Next, FIG. 18 shows anembodiment for improving the estimation of the wheel factor, with acorrection 62 made by a feed back of sensor signals such as lateralacceleration and yaw rate, on the basis of the observer 61 using thevehicle model. And, FIG. 19 shows a further embodiment for calculatingthe wheel factor Wx directly, by means of a state amount calculation 63on the basis of the steering angle of the wheel, vehicle speed, lateralacceleration, yaw rate and the like, without using the observer asdescribed above. Furthermore, more than two estimation units out of theplurality of estimation units may be performed in parallel, to obtainthe wheel factor Wx with each estimated result weighted, respectively.

In the embodiments as described above, the grip factor ε was obtained onthe basis of the aligning torque, in view of variation of the pneumatictrail of tire. Whereas, on the basis of a margin of side force for roadfriction, a grip factor indicative of a grip level of the tire in itslateral direction can be estimated (in this case “εm” is used herein),as described hereinafter.

According to a theoretical model of a tire, so-called brush model, whichis used for analyzing the force produced on the tire, the relationshipbetween the (actual) aligning torque Tsaa to the (front) side force Fycan be obtained in accordance with the following equations:Provided that ξ=1−{Ks/(3·μ·Fz)}·λ,If ξ>0, Fy=μ·Fz(1−ξ³)  (1)If ξ≦0, Fy=μ·Fz  (2)And,If ξ>0, Tsaa=(1·Ks/6)·λ·ξ³  (3)If ξ≦0, Tsaa=0  (4)where “Fz” is the vertical load, “l” is the length of the tire surfacecontacting the road, “Ks” is a constant corresponding to the treadhardness, “λ” is the side slip (λ=tan α), and “α” is the wheel slipangle.

In general, the slip angle α is small in the area of ξ>0, the equationof λ=α may be applied. As apparent from the equation (1), the maximalvalue of the side force is μ·Fz. Therefore, if a portion of side forceaccording to the road coefficient of friction μ to the maximal value ofside force is indicated by a coefficient of friction utilization ratioη, then the ratio η can be given by η=1−ξ³. Therefore, εm=1−η means amargin for (road) coefficient of friction, so that the grip factor εmcan be given by εm=ξ³. As a result, the equation (3) can be rewritten bythe following equation:Tsaa=(1·Ks/6)·α·εm  (5)

The equation (5) indicates that the aligning torque Tsaa is proportionalto the slip angle α and the grip factor εm. Then, if the characteristicobtained when εm=1 (the utilization ratio of coefficient of friction iszero, and the margin for coefficient of friction is 1) is used for thereference aligning torque characteristic, the reference aligning torqueTsau is given by the following equation:Tsau=(1·Ks/6)·α  (6)

Then, the grip factor εm can be obtained by the equations (5) and (6) asfollows:εm=Tsaa/Tsau  (7)

In the equation (7), the road coefficient of friction μ is not includedas the parameter. Thus, the grip factor εm can be calculated withoutusing the road coefficient of friction μ. In this case, the gradient K4(=1·Ks/6) of the reference aligning torque Tsau can be set in advance bymeans of the brush model, or can be obtained through experiments.Furthermore, if the initial value is set at first, then the gradient ofthe aligning torque is identified in the vicinity of the origin of theslip angle when the vehicle is running, to correct the initial value,the accuracy of the grip factor will be improved.

As shown in FIG. 20, for example, if the slip angle is α2, the referencealigning torque Tsau2 is given by Tsau2=K4·α2. And, the grip factor εmcan be obtained by the following equation:εm=Tsaa 2/Tsau 2=Tsaa 2/(K 4·α2)

Accordingly, in lieu of the grip factor ε based on the pneumatic trailas described in FIGS. 14-23, the grip factor εm based on the margin ofside force for road friction can be employed. The relationship betweenthose grip factors ε and εm will be the one as shown in FIG. 21.Therefore, after the grip factor ε was obtained, then it may beconverted into the grip factor εm. On the contrary, after the gripfactor εm was obtained, then it may be converted into the grip factor ε.

Next will be explained an embodiment of the coefficient of frictionestimation apparatus for estimating the road coefficient of friction μon the basis of the aligning torque and the wheel factor such as theside force or wheel slip angle. FIG. 22 shows an embodiment of thecoefficient of friction estimation apparatus, wherein the reaction forcetorque is calculated by the motor current at units M21-M25, in the samemanner as the grip factor estimation apparatus as shown in FIG. 7 (InFIG. 22, “20” has been added to the number following “M” in each unit ofFIG. 7) and the frictional torque in the steering system is adjusted, toestimate the aligning torque. The wheel factor is obtained through unitsM26-28 in the same manner as the blocks disclosed in FIGS. 17-19. Then,the road coefficient of friction μ is obtained at a coefficient offriction estimation unit 30, on the basis of the relationship betweenthe wheel factor and the aligning torque.

FIG. 23 shows an example of the coefficient of friction estimation unitM30, wherein the coefficient of friction is estimated on the basis ofthe aligning torque estimated at the aligning torque estimation unit M25and the wheel factor estimated at the wheel factor estimation unit M28.At a unit M31, the grip factor ε is estimated on the basis of thealigning torque Tsa and the wheel factor Wx, as shown in FIGS. 7-14. Ata unit M33 for estimating the road coefficient of friction, the roadcoefficient of friction A is estimated on the basis of the aligningtorque and the wheel factor which are obtained when the grip factor hasreached a predetermined reference grip factor set at the reference gripfactor setting unit M32 for determining the road coefficient of frictionto estimate. As the wheel factor is affected by the vehicle behavior,the value indicative of the vehicle behavior obtained when reached thereference grip factor, i.e., lateral acceleration or yaw rate, may beused, in stead of the value indicative of the vehicle behavior.

Referring to FIG. 24, an example for estimating the road coefficient offriction p when the side force Fy is used as the wheel factor Wx will beexplained hereinafter. In FIG. 24 shows the relationship between theside force Fy and the aligning torque Tsa when the road coefficient offriction μ was lessened, wherein a solid line indicates thecharacteristic at a high μ and a broken line indicates thecharacteristic at a low μ. When the shape of the area of the roadcontacting the tire and elasticity of the tread rubber are constant, thecharacteristic of the side force to the aligning torque is analog to theroad coefficient of friction μ (the characteristic of solid line andbroken line in FIG. 24). Therefore, the side force Fy or the aligningtorque Tsa provided when the grip factor ε obtained by a ratio betweenthe reference aligning torque and the actual aligning torque isidentical, directly reflects the road coefficient of friction μ.

Accordingly, the grip factor ε obtained at the high μ is indicated bythe equation of ε=Line segment [J-Fy1]/Line segment [H-Fy1], and thegrip factor ε′ obtained at the low μ is indicated by the equation ofε′=Line segment [J′-Fy2]/Line segment [H′-Fy2], so that a triangle[0-H-Fy1] is analogue to a triangle [0-H′-Fy2]. In case of ε=ε′,therefore, the ratio of Line segment [0-Fy1] to Line segment [0-Fy2],i.e., the ratio of the aligning torque Tsaa1 to the aligning torqueTsaa2, corresponds to the ratio of the road coefficient of friction μ.As a result, by setting a certain grip factor on a dry asphalt roadsurface (μ=approximately 1.0) as a reference, it is possible to estimatethe road coefficient of friction μ on the basis of the side force Fy oraligning torque Tsa for providing the certain grip factor. Referring toFIG. 24, therefore, the road coefficient of friction can be estimated onthe basis of the value of side force (Fy1, Fy2) or aligning torque(Tsaa1, Tsaa2) obtained when reached the reference grip factor (points Jand J′) in FIG. 24.

Likewise, the road coefficient of friction μ can be estimated when thewheel slip angle α is used as the wheel factor Wx, as will be explainedhereinafter with reference to FIG. 25. In this case, the aligning torqueTsa has the nonlinear characteristic to the wheel slip angle α, asexplained before with respect to the estimation of the grip factor.Therefore, the characteristic of the aligning torque to the wheel slipangle is approximated to a linear characteristic as indicated bytwo-dotted chain line in FIG. 25, to estimate the road coefficient offriction μ in a linear zone (0-M zone) to the wheel slip angle α.

FIG. 26 shows the relationship between the wheel slip angle α and thealigning torque Tsa, as that in FIG. 25, wherein a solid line indicateswhen the coefficient of friction μ is high and a broken line indicateswhen the coefficient of friction μ is low. As apparent from FIG. 26, thecharacteristic of the wheel slip angle to the aligning torque is analogto the road coefficient of friction μ (the characteristic of solid lineand broken line in FIG. 26), similar to that in FIG. 24. Therefore, theroad coefficient of friction can be estimated on the basis of the valueof aligning torque or wheel slip angle (α1, α2) obtained when reachedthe reference grip factor (points S and S′ in FIG. 26) set in advance.In this case, the reference grip factor is required to be set in a zonewhere the relationship between the wheel slip angle and the side forceis in a linear characteristic. Whereas, in order to estimate the roadcoefficient of friction accurately, it is required to be estimated in azone where a certain difference will be caused between the referencealigning torque and the actual aligning torque. In view of thoserequirements, therefore, it is preferable to set the reference gripfactor experimentally in such a state that the road coefficient offriction is relatively high as on the dry asphalt road surface, forexample.

In the case where the estimation of the road coefficient of friction ismade on the basis of the grip factor, in lieu of the grip factor ε basedon the pneumatic trail, the grip factor εm based on the margin of sideforce for road friction can be employed. As the relationship betweenthose grip factors ε and εm will be the one as shown in FIG. 21, afterthe grip factor ε was obtained, then it may be converted into the gripfactor εm, whereas, after the grip factor εm was obtained, then it maybe converted into the grip factor ε.

According to the road condition estimation apparatus as constitutedabove, the grip factor and coefficient of friction can be easilyestimated, because the steering control system of the present inventionis based on the steer-by-wire system. With respect to the apparatus withthe manually operated steering member mechanically connected to thewheels to be steered, it is required to detect separately the torqueproduced by manipulation of the vehicle driver, and the torque producedby the steering assist apparatus (so-called power steering apparatus).In contrast, according to the steer-by-wire system, the torque outputfrom the actuating device (motor) and the reaction torque of the wheelreceived from the road surface are substantially coincide with eachother, the actuating device can be used as a sensor for estimating theroad condition. The output torque can be obtained by detecting theelectric current for actuating the motor MF (MR). And, the currentdetecting section 23 is required for the control of the motor MF (MR)and a failsafe. Therefore, the road conditions including the grip factorand the coefficient of friction can be estimated easily, so thatreduction in cost can be easily achieved.

Next will be explained the vehicle motion control apparatus having theroad condition estimation apparatus for estimating the grip factor orthe road coefficient of friction as described above. FIG. 27 shows asetting process of a desired steering angle of the front wheels of thevehicle with the steer-by-wire system. At a steering operation detectionunit M41, the state of steering operation (steering wheel operationangle) by a vehicle driver is detected. A steering ratio between asteering wheel operation angle and a steering angle of a wheel is set ata front wheel steering ratio setting unit M45, on the basis of the stateof steering operation detected at the steering operation detection unitM41, a vehicle speed detected at a vehicle speed detection unit M42, atleast one of the grip factor and road coefficient of friction estimatedat a grip factor and coefficient of friction estimation unit M43,wherein the grip factor and the road coefficient of friction areestimated in accordance with the process as described before.Accordingly, a desired value of a steering angle for front wheels isdetermined on the basis of the steering ratio set at the front wheelsteering ratio setting unit M45 and the steering wheel operation angledetected at the steering operation detection unit M41.

The steering ratio for the front wheels is set at the front wheelsteering ratio setting unit M45, so as to be large when the vehiclespeed is relatively low, and small when the vehicle speed is relativelyhigh. Therefore, convenience in arranging the steering system on thevehicle will be improved, because the steering angle of the front wheelcan be obtained with the steering wheel operated by a small amount, whenthe vehicle speed is low. On the contrary, the vehicle stability will beimproved, because the steering angle of the front wheel is maderelatively small in response to the operation of the steering wheel,when the vehicle speed is high. In addition, when the speed of operationof the steering wheel is fast (i.e., steering wheel operation angularvelocity is large), the steering ratio for the front wheels is set to belarger than that in a normal steering operation. Consequently, thevehicle maneuverability will be improved, in such a case that thevehicle is required to be immediately turned to avoid an obstacle on theroad. The steering ratio for the front wheels is set on the basis of atleast one of the grip factor and the road coefficient of friction. Whenat least one of the grip factor and the road coefficient of friction isestimated to be relatively low, the steering ratio for the front wheelsis set to be small. Consequently, when the road coefficient of frictionis low, or when the grip factor has become low, the steering angle ofthe front wheel is set to be relatively low in response to operation ofthe steering wheel, an excessive steering angle will not be give to thefront wheels, so that the vehicle stability will be improved.

Furthermore, a modified steering angle for making the vehicle behaviorstable is added to the steering angle, when setting the desired steeringangle in response to operation of the steering wheel by a vehicledriver, so that a final desired steering angle θft of the front wheel isset at a vehicle stability control unit M48. The modified steering angleis determined by a deviation between a reference vehicle state variableobtained on the basis of the amount of steering wheel operation and thevehicle speed at a reference vehicle state variable unit M46, and theactual vehicle state variable calculated by a vehicle state variablecalculation unit M47 the result detected by a vehicle behavior detectionunit M44, as shown in FIG. 27. As a result, the road coefficient offriction or the grip factor is reflected in the reference vehicle model,to be made more accurate. In addition, when the modified steering angleis determined at the vehicle stability control unit M48, at least one ofthe road coefficient of friction and the grip factor is used. Forexample, when the road coefficient of friction is low, or when the gripfactor is decreased, the threshold level for initiating the vehiclestability control can be set lower than that in a normal vehicle state,and the amount to be controlled on the basis of the deviation of thevehicle state variable can be set lower.

The desired steering angle of the rear wheels is set according to theblocks as shown in FIG. 28, in the same manner as the setting process ofthe desired steering angle of the front wheels as described before. Asthe blocks as shown in FIG. 28 is constituted in substantially the sameas those disclosed in FIG. 27, their explanations are omitted herein,with the last reference letters corresponding to each other omitted inFIG. 28. Accordingly, a steering ratio between a steering wheeloperation angle and a steering angle of rear wheels is set on the basisof at least one of the state of steering operation (steering wheeloperation angle), the vehicle speed, and at least one of the grip factorand road coefficient of friction. The steering ratio of the rear wheelsis controlled to set the steering operation in an opposite phase (in theopposite direction to the steering wheel operation) when the vehiclespeed is low, and in a common phase(in the same direction to thesteering wheel operation) when the vehicle speed is high, on the basisof at least one of the vehicle speed, and at least one of the gripfactor and road coefficient of friction. Consequently, the steer abilityis improved when the vehicle speed is low, and the vehicle stability isimproved when the vehicle speed is high. As the steering ratio of therear wheels is set appropriately on the basis of the road coefficient offriction or the grip factor, the vehicle stability can be improvedfurther. In the case where the steering wheel is operated fast (i.e.,the steering angular velocity is high) in case of an emergency such aspresence of obstacles ahead of the vehicle, it is possible to controlthe steering apparatus to be in an opposite phase for a moment by aso-called phase inversion control, even if the vehicle is running athigh speed, to improve the vehicle maneuverability. In the case wherethe road coefficient of friction is low, or the grip factor isdecreased, however, it is preferable to prohibit the phase inversioncontrol, so as to ensure the vehicle stability.

In order to ensure a safety against disturbance or the like, the desiredrear wheel angle is set in accordance with the vehicle state variable.That is, a modified rear wheel angle is determined by a deviationbetween a reference vehicle state variable obtained on the basis of theamount of operation of the steering wheel and the vehicle speed, and theactual vehicle state variable calculated by a vehicle state variablecalculation unit M57. In this case, the road coefficient of friction orthe grip factor is reflected in the reference vehicle model, to be mademore accurate. In addition, when the modified steering angle isdetermined at the vehicle stability control unit M58, at least one ofthe road coefficient of friction and the grip factor is used. Forexample, when the road coefficient of friction is low, or when the gripfactor is decreased, the amount to be controlled on the basis of thedeviation of the vehicle state variable can be set lower.

FIG. 29 shows a block diagram for setting a desired steering reactionforce at the steering reaction force simulator (SST) as shown in FIGS. 3and 4. It is required for the steering reaction force simulator (SST) toprovide the appropriate reaction force, and the information about thereaction force applied to the wheel from the road surface. In order tocomply with the requirement, it is so constituted at the desiredsteering reaction force setting unit M67 that the desired steeringreaction force rt is set in accordance with at least one of the amountof operation of the steering wheel, steering angle of the wheel, vehiclespeed, vehicle state variable, grip factor, and coefficient of friction,to provide a characteristic of steering reaction in accordance with theamount of operation of the vehicle driver. As shown in FIG. 29, thesteering operation detection unit M61, steering angle of wheel detectionunit M62, grip factor and coefficient of friction estimation unit M63,vehicle speed detection unit M64, vehicle behavior detection unit M65and the vehicle state variable calculation unit M66 are provide fordetecting, estimating, and calculating the factors in the same manner asdescribed before. And, at the desired steering reaction force settingunit M67, when the vehicle is running at high speed, the desiredsteering reaction force is set to be increased, so as to improve thevehicle stability. When at least one of the grip factor and thecoefficient of friction is decreased, the desired steering reactionforce is set to be different from that in the normal condition, e.g.,larger or smaller than the normal steering reaction force, and thisstate is informed to the vehicle driver.

FIG. 30 shows a block diagram for setting a desired braking force foreach wheel. At a wheel braking force setting unit M77, on the basis ofthe amount of braking operation, vehicle speed, and at least one of thegrip factor and the road coefficient of friction, desired values (Bfrt,Bflt, Brrt, Brlt) of braking force for each wheel are set for the normalmode (i.e., other than the control modes such as ABS, TRC, EBD, VSC, orBA as described before). Thus, the characteristic of the desired brakingforce is set in accordance with the amount of operation of the vehicledriver. As shown in FIG. 30, the vehicle speed detection unit M71, brakepedal operation detection unit M72, grip factor and coefficient offriction estimation unit M73, wheel speed detection unit M74, steeringangle of wheel detection unit M75 and vehicle behavior detection unitM76 are provide for detecting an estimating the factors in the samemanner as described before. And, at a wheel braking force setting unitM77, when the vehicle is running at high speed, the braking forceapplied to the front wheels is set to be larger than that applied to therear wheels in the braking force distribution control, so as to ensurethe vehicle stability. Also, on the basis of at least one of the gripfactor and the road coefficient of friction, the desired wheel brakingforce for each wheel is set as shown in FIG. 31.

In the case where the road coefficient of friction or the grip factor ishigh, it is set as indicated by a solid line O-A-B in FIG. 31. In thecase where the amount of operation of the brake pedal is small, i.e.,the vehicle deceleration is small, it is set as indicated by a linesegment O-A in FIG. 31, to increase the gradient of the desired brakingforce in response to the amount of operation of the brake pedal. Whenthe amount of operation of the brake pedal has increased to a certainamount, i.e., when the vehicle deceleration has become large, thegradient is set to be small as indicated by a line segment A-B, so thatcontrollability of the vehicle deceleration to the amount of operationwill be improved. In the case where the road coefficient of friction orthe grip factor is low, the upper limit of the vehicle deceleration tobe obtained is limited, so that a dead part will be caused in theoperation of the brake pedal. Therefore, the gradient of the desiredbraking force to the amount of operation of the brake pedal is set to besmall, as indicated by a broken line in FIG. 31, so that thecontrollability of the vehicle deceleration will be improved. In thiscase, if an error was caused in estimating the road coefficient offriction or the grip factor, the gradient of the desired braking forceto the amount of operation of the brake pedal should be set to be large,as indicated by a line segment C-B in FIG. 31, so as to ensure a maximalvehicle speed as a fail safe.

Furthermore, in order to improve the vehicle stability or the vehicledeceleration property, the desired braking force for each wheel ismodified at the braking force control unit M78, wherein the brakingforce control is performed, such as the anti-skid control (ABS),traction control (TRC), vehicle stability control (VSC), braking forcedistribution control (EBD), brake assist control (BA) and the like,which are generally known. Therefore, it is so constituted that thethreshold values for determining to initiate or terminate thosecontrols, or the controlling amount for those controls are determined onthe basis of at least one of the grip factor and the road coefficient offriction.

In the warning system ALM, it is so constitute that if at least one ofthe estimated grip factor and the road coefficient of friction isdecreased to a predetermined threshold level, a warning information isgiven to the vehicle driver by means of a warning apparatus (as shown inFIG. 4) through sound, voice, light, vibration or the like. Furthermore,when the estimated grip factor is reduced to be less than apredetermined value, for example, it may be so constituted that theengine output shall be reduced, or the braking effect through enginebrake shall be increased by a shift down, to brake the vehicleautomatically, so that the vehicle speed will be reduced.

It should be apparent to one skilled in the art that the above-describedembodiment are merely illustrative of but a few of the many possiblespecific embodiments of the present invention. Numerous and variousother arrangements can be readily devised by those skilled in the artwithout departing from the spirit and scope of the invention as definedin the following claims.

1. An apparatus for estimating a road condition for use in a vehiclehaving steering control means for actuating a device mechanicallyindependent of a manually operated steering member to steer at least awheel of front and rear wheels of said vehicle, comprising: actuatingsignal detection means for detecting an actuating signal for-actuatingsaid device of said steering control means; aligning torque estimationmeans for estimating an aligning torque produced on said wheel on thebasis of the actuating signal detected by said actuating signaldetection means; vehicle state variable detection means for detecting astate variable of said vehicle; wheel factor estimation means forestimating at least one of wheel factors including a side force and aslip angle applied to said wheel on the basis of the state variabledetected by said vehicle state variable detection means; and grip factorestimation means for estimating a grip factor of at least a tire of saidwheel, in accordance with a relationship between the aligning torqueestimated by said aligning torque estimation means and the wheel factorestimated by said wheel factor estimation means.
 2. An apparatus forestimating a road condition as set forth in claim 1, further comprising;reference aligning torque setting means for setting a reference aligningtorque on the basis of the wheel factor estimated by said wheel factorestimation means and the aligning torque estimated by said aligningtorque estimation means, wherein said grip factor estimation meansestimates the grip factor of said tire on the basis of a result ofcomparison between the aligning torque estimated by said aligning torqueestimation means and the reference aligning torque set by said referencealigning torque setting means.
 3. An apparatus for estimating a roadcondition as set forth in claim 2, wherein said reference aligningtorque setting means sets the reference aligning torque by approximatinga characteristic of the aligning torque estimated by said aligningtorque estimation means against the wheel factor estimated by said wheelfactor estimation means to a linear characteristic of the referencealigning torque including at least the origin, and sets the referencealigning torque on the basis of the linear characteristic of thereference aligning torque.
 4. An apparatus for estimating a roadcondition as set forth in claim 2, wherein said reference aligningtorque setting means sets a linear characteristic of the referencealigning torque with a gradient which is provided by a brush model ofsaid wheel, and sets the reference aligning torque on the basis of thelinear characteristic of the reference aligning torque.
 5. An apparatusfor estimating a road condition as set forth in claim 1, furthercomprising; friction estimation means for estimating a coefficient offriction of a road on which said vehicle is running, on the basis of thegrip factor estimated by said grip factor estimation means.
 6. Anapparatus for estimating a road condition as set forth in claim 5,further comprising; warning means for warning to a vehicle driver whenat least one of road factors including the grip factor estimated by saidgrip factor estimation means and the coefficient of friction estimatedby said friction estimation means is less than a predetermined value. 7.A vehicle motion control apparatus having an apparatus for estimating aroad condition for use in a vehicle having steering control means foractuating a device mechanically independent of a manually operatedsteering member to steer at least a wheel of front and rear wheels ofsaid vehicle, comprising: actuating signal detection means for detectingan actuating signal for actuating said device of said steering controlmeans; aligning torque estimation means for estimating an aligningtorque produced on said wheel on the basis of the actuating signaldetected by said actuating signal detection means; vehicle statevariable detection means for detecting a state variable of said vehicle;wheel factor estimation means for estimating at least one of wheelfactors including a side force and a slip angle applied to said wheel onthe basis of the state variable detected by said vehicle state variabledetection means; grip factor estimation means for estimating a gripfactor of at least a tire of said wheel, in accordance with arelationship between the alignment torque estimated by said aligningtorque estimation means and the wheel factor estimated by said wheelfactor estimation means; and control means for performing at least oneof a steering control to front wheels of said vehicle, a steeringcontrol to rear wheels of said vehicle and a braking force control toeach wheel of said vehicle, on the basis of the grip factor estimated bysaid grip factor estimation means.
 8. A vehicle motion control apparatusas set forth in claim 7, further comprising; friction estimation meansfor estimating a coefficient of friction of a road on which said vehicleis running, on the basis of the grip factor estimated by said gripfactor estimation means, wherein said control means performs at leastone of the steering control to front wheels of said vehicle, thesteering control to rear wheels of said vehicle and the braking forcecontrol to each wheel of said vehicle, on the basis of at least one ofroad factors including the grip factor estimated by said grip factorestimation means and the coefficient of friction estimated by saidfriction estimation means.
 9. A vehicle motion control apparatus as setforth in claim 8, wherein said control means provides parameters for atleast one of the steering control to front wheels of said vehicle, thesteering control to rear wheels of said vehicle and the braking forcecontrol to each wheel of said vehicle, on the basis of at least one ofthe road factors including the grip factor and the coefficient offriction.
 10. A vehicle motion control apparatus as set forth in claim8, wherein said control means performs a reaction force control to saidmanually operated steering member, and wherein said control meansprovides a parameter for said reaction force control, on the basis of atleast one of the road factors including the grip factor and thecoefficient of friction.
 11. A vehicle motion control apparatus as setforth in claim 8, wherein said control means provides a characteristicof steering amount of wheel in accordance with the amount of steeringoperation of a vehicle driver for the steering controls to front wheelsand rear wheels of said vehicle, on the basis of at least one of theroad factors including the grip factor and the coefficient of friction,and wherein said control means provides a characteristic of a desiredbraking force in accordance with the amount of braking operation of thevehicle driver for the braking force control to each wheel of saidvehicle, on the basis of at least one of the road factors including thegrip factor and the coefficient of friction.
 12. A vehicle motioncontrol apparatus as set forth in claim 7, further comprising; referencealigning torque setting means for setting a reference aligning torque onthe basis of the wheel factor estimated by said wheel factor estimationmeans and the aligning torque estimated by said aligning torqueestimation means, wherein said grip factor estimation means estimatesthe grip factor of said tire on the basis of a result of comparisonbetween the aligning torque estimated by said aligning torque estimationmeans and the reference aligning torque set by said reference aligningtorque setting means.
 13. A vehicle motion control apparatus as setforth in claim 12, wherein said reference aligning torque setting meanssets the reference aligning torque by approximating a characteristic ofthe aligning torque estimated by said aligning torque estimation meansagainst the wheel factor estimated by said wheel factor estimation meansto a linear characteristic of the reference aligning torque including atleast the origin, and sets the reference aligning torque on the basis ofthe linear characteristic of the reference aligning torque.